As is well known, various types of electronic devices have currently been proposed and supplied for practical use. Some examples include IC packages loaded with an IC chip, and crystal oscillators loaded with a crystal resonator. These electronic devices are normally produced by mounting an electronic element in a housing (also referred to as a case or casing) composed of an insulating substrate and its cover, and then sealing the housing. Examples of materials used for the insulating substrate include glass, ceramics, silicon and other inorganic materials.
More recently, multiple production systems have been proposed for producing electronic devices in large volume. The use of such a system makes it possible to simultaneously produce a large number of electronic devices in a single production process by preparing a single large inorganic material substrate, fabricating a large number of housings in that substrate, mounting electronic elements and then individually cutting away the finished electronic devices.
For example, a method for simultaneously producing a large number of piezoelectric vibrators is disclosed in Japanese Unexamined Patent Publication No. 60-187117 by composing a casing by sealing first and second glass substrates each provided with cavities, and then arranging piezoelectric elements inside that casing. According to this production method, after forming a large number of cavities in each glass substrate by chemical etching and then attaching the piezoelectric elements, the second glass plate is superimposed so that corresponding cavities are aligned followed by adhering by heating in a vacuum and sealing. Finally, the two sealed glass substrates are scribed, enabling a large number of piezoelectric vibrators to be obtained by breaking along the scratched lines.
In addition, a method for producing piezoelectric vibrators is also disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-121985. According to this production method, a large number of piezoelectric vibrators can be obtained by joining a first substrate mounted with a plurality of piezoelectric vibrator pieces and a second substrate in opposition to this substrate to form a joined substrate with the piezoelectric vibrator pieces sealed inside followed by cutting the joined substrates. Silicon wafers or glass substrates are used for the substrates, and selective etching is used to form the cavities for housing the piezoelectric vibrator pieces.
As has been described above, in the case of conventional methods for producing piezoelectric vibrators using a multiple production system, although there are examples of using silicon and glass for the material of the housing in which cavities are formed, examples of the use of ceramics are not widely known. This is because, in order to form a housing from ceramics, since the ceramics must rapidly be processed into the housing or a form close to it in the state of a non-shrunken green sheet prior to baking, and after forming wiring of a high-temperature metal material in a prescribed pattern, must be baked at a high temperature of about 1550-1650° C., dimensional shrinkage of about 20% cannot be avoided. When this degree of dimensional shrinkage is present in the substrate after baking, variations occur in the shape and dimensions of the resulting housing, and a decrease in yield cannot be avoided.
The problem of the dimensional shrinkage described above that occurs in the case of producing housings from ceramics becomes increasingly serious as the size of the substrate after baking becomes larger. For example, although Japanese Unexamined Patent Publication (Kokai) No. 11-340350 proposes a method for forming a plurality of housings by laminating ceramic green sheets and forming a plurality of cavities followed by baking and superimposing an integrated lid corresponding to the location of each cavity to seal the cavities all at once, this method is unable to solve the problem caused by dimensional shrinkage of about 20m% that occurs during baking. In other words, this is because, in order to form a plurality of housings simultaneously, it is necessary to use a greater dimensional margin, and in the case of loading large substrates after baking into a processing device or mounting device for multiple production, continuous index processing is no longer possible due to the poor dimensional accuracy among the plurality of housings.
In order to explain this problem, the following provides an explanation of a method of the prior art for producing crystal vibrators by a multiple production system using ceramics for the substrate material.
First, as is respectively shown in FIGS. 1A and 1B, two types of regular size sheets are prepared. In order to accomplish this, a long green sheet of regular size width is formed from a slurry containing ceramic powder, binder and so forth. A regular size sheet 2b (FIG. 1B) is then blanked and punched from this long sheet to dimensions based on a preset shrinkage factor, and at that time, work reference holes 27 are punched for lamination treatment. Next, cavities 12 for housing electronic elements are punched out using work reference holes 27 of regular size sheet 2b as a reference to form regular size sheet 2a (FIG. 1A). Continuing, a prescribed number of regular size sheets 2a and 2b are laminated and pressed together in predetermined combinations. The lamination step can be carried out by, for example, pressing together for a fixed amount of time at 200-250° C. FIG. 2 indicates a laminated green sheet 2 obtained in this manner. In this drawing, breaking lines 29 fabricated in a later step are also suitably indicated to facilitate understanding.
Next, the resulting laminated green sheet 2 is processed in order as shown in FIGS. 3A through 3E, which are each cross-sectional views taken along lines III—III of FIG. 2.
First, through holes 13 for connecting the top and bottom sides are opened as shown in FIG. 3A using work reference holes 27 of the resulting laminated green sheet 2 as a reference.
Next, as shown in FIG. 3B, top wiring layers 21 and 26, bottom wiring layers 22 and electrical connections 23 are formed in the laminated green sheet 2. To complete this step, a high melting point metal such as W or Mo is filled into through holes 13 opened in the previous step to obtain electrical connections 23. Next, top wiring layers 21 and 26 and bottom wiring layers 22 are formed by applying a coating of a high melting point metal such as W or Mo. Subsequently, notches are made to a depth of about 50% of the thickness of laminated green sheet 2 with a die to form breaking lines 29. Breaking lines 29 are for separating the ceramic substrate in electronic device units during the production process. Subsequently, laminated green sheet 2 is baked at 1550-1650° C. in a hydrogen atmosphere. Dimensional shrinkage of 20% occurs during this baking resulting in the completion of ceramic substrate 2 in which is formed cavities, through holes, wiring layers and breaking lines.
Continuing, ceramic substrate 2 is cut apart at the locations of breaking lines 29. As shown in FIG. 3C, ceramic substrates 2 are obtained each corresponding to units of the desired electronic devices. Namely, this ceramic substrate 2 can be said to be a precursor of the electronic devices.
Next, as shown in FIG. 3D, ceramic substrates 2 are loaded into a carrier (jig) 30 and transported to the final production step shown in FIG. 3E. First, crystal resonator 1 is mounted in cavity 12 formed in ceramic substrate 2. In order to perform this mounting, an electrically conductive paste 24 is coated onto wiring layer 21 on ceramic substrate 2, and crystal resonator 1 is mounted followed by heating. For example, the heating temperature is about 180° C. when the electrically conductive paste is a thermosetting resin containing Ag.
After crystal resonator 1 has finished being mounted, a lid member.(“kovar” is used here) 3 is superimposed on the upper surface of ceramic substrate 2 and sealed to be airtight. In order to perform this airtight sealing, after arranging a foil-shaped low-temperature metal brazing material (an Au—Sn alloy brazing material is used here) at the joined locations of ceramic substrate 2 to form junction layer 25, ceramic substrate 2 and lid member 3 are brazed by means of junction layer 25. Crystal vibrator 1 fabricated in this manner is then shipped to the market following final inspection.
However, the above production method and other production methods based on a multiple production system have numerous problems. For example, although a plurality of cavities are formed on a ceramic substrate, since the positional accuracy of each cavity is poor, a plurality of lid members are unable to be simultaneously integrated into a single unit, thereby making it necessary to mount the electronic elements and seal the lid members after separating the cavities into individual units and loading into a jig, and increasing the complexity of the production process. In addition, there is also the problem of cuttings falling into the cavities when each of the cavities are separated into individual units, as well as the problem of debris falling into the cavities at the stage the ceramic substrate containing cavities is loaded into the jig.